Try using DateDiff in the update to.

Also, both those fields are in that table, right? and are date fields?

Also Also, this is something that could probably just be left as a query calculation and not need to be a value stored in the table. Unless you have some reason to do so :P.

Yes both in the table and date fields.
I watched the DateDiff video and put DateDiff("d",[ArrivalDate],[DepartureDate]) in the Update to, but got the same result.
I need the number of days to calculate the cost.

Try
CDate([DepartureDate])-CDate([ArrivalDate])

Still the same result

Can we see the values of those two fields normally? Are they just a regular 10-28-2022 date? That makes me think it's either not seeing it as a date field or that the date field is formatted in a way it can't subtract or convert.

Also what type of field is NumDays?

What is that format :P that's your problem

Take that format \[d"eparture"d"ate]-[Arrival]" out all togethor

Something definitely weird going on.
I change the date format back t dd/mm/yyyy.
The i checked Numdays format, very weird.
Just got rid of that hocus pocus and it is working.
Sorry to have wasted your time Adam.

No problem :)

How can that just randomly happen?
I certainly never put that in the format!!
Is this another "Evil Access Stuff"?

That kind of stuff from what I've seen is usually just a freak accident user error. Maybe coding something with some typos and went to erase it and maybe hit some key to copy or paste it and that happened, or maybe accidentally drag and dropped some text somehow. Or maybe a combination of a few things, who knows, I've never seen it randomly change tables that have been around for a long time and aren't edited anymore though so I think it has to be some user/windows accident. I don't doubt you though that you didn't type that out purposely, that's a pretty random thing of characters haha.